The multi-anode microchannel array (MAMA) detector employs a photocathode for photon/electron conversion, a microchannel plate (MCP) for electron multiplication and a proximity focused anode array combined with charge amplifiers for event detection. Digital decoding electronics interpret the charge amplifier outputs to determine the pixel position of an event. The block diagram of a prior art MAMA detector is shown in FIG. 1.
The current generation of prior art MAMA detectors employ arrays which consist of two sets of interleaved anodes in a repeating series (see FIG. 2) which for historical reasons are called "fine-fine" anode arrays. The first set of anodes is on top and consists of n anodes (which repeats for n+2 cycles) and the second set is on the bottom and consists of n+2 anodes (which repeats for n cycles). This results in a total of n*(n+2) pixels, where n must be even to insure unique decoding over the entire array. Another pair of anode sets (not shown in FIG. 2) of m and m+2 anodes run underneath and perpendicular to the first pair of anode sets, resulting in a total of m*(m+2) pixels in the perpendicular axis. A pixel is defined as spanning from one anode's center line to the next anode's center line.
The configuration of a fine-fine array requires a complex algorithm involving coincidence discrimination for determining the position of a given photon event. Coincidence discrimination is the process of taking two or more anodes which experience electron pulses which are coincident in time and inferring the pixel location of the event in the anode array from the combination of anodes. While coincidence discrimination requires a more complex position decoding algorithm than would be needed for a configuration of discrete anodes (i.e. a separate and unique anode for each pixel), it requires far fewer anodes and therefore far fewer charge amplifiers.
The size of the electron cloud due to a single photon event varies depending on the characteristics of the microchannel plate or, simply, MCP as well as bias voltages applied to the photocathode, MCP and anode array. The electron cloud diameter is quantized by the total number of anodes illuminated in a given axis, also referred to as the order of the fold. For example, a three-fold designates the situation in which three contiguous anodes are struck by sufficient numbers of electrons to have voltages greater than some user-specified threshold. A scaled-down version of one axis of a fine-fine anode array (n=4) with one-, two-, three-, and fourfolds is shown in FIG. 2. The electron cloud diameter must be sufficient to illuminate at least two anodes (a two-fold) in order to allow for the unique decoding of the position of the event. Owing to size variations in the electron cloud emanating from the MCP, the decoding algorithm must be capable of coping with higher-order folds. As FIG. 2 illustrates, every higher-ordered fold can be reduced to an equivalent two-fold (that two-fold which occupies the same pixel as the higher-ordered fold). Notice that in .the case of a three-fold (or any odd-fold) there are two possible equivalent two-folds. A decoder's function is to take an arbitrarily ordered fold and infer the corresponding pixel position of the event. Since there is one decoder input for each anode, a single axis requires 2n+2 inputs. A typical value for n is 32, in which case the decoder must accommodate 66 inputs or a single axis of the detector. Because of the large number of inputs and complicated requirements for legal events, event decoding cannot easily be reduced to a single step process. The algorithm for event decoding is therefore divided into two stages: anode encoding, which is the process of converting a k-fold into the equivalent two-fold for arbitrary k (see FIG. 2); and pixel decoding, which is the process of translating the equivalent two-fold into the corresponding pixel position.